1. Field of the Invention
The present invention relates to a machine structure steel and a process for production thereof, said steel being useful as a raw material to be made into parts of industrial machines, automobiles, and electric appliances by machining. More particularly, the present invention relates to a machine structure steel and a process for production thereof, said steel being superior in chip disposability and mechanical properties despite its substantial freedom from lead (Pb) as a machinability improving component.
2. Description of the Related Arts
Good machinability is required of a steel to be made into parts of industrial machines, automobiles, and electric appliances by machining. A conventional way to improve the machinability of a machine structure steel for such parts is to incorporate the steel with lead (Pb) or sulfur (S) as a machinability improving component. It is known that lead (Pb) even in a small amount greatly improves machinability.
Japanese Patent Laid-open No. 205453/1984 discloses a free machining steel which is incorporated with S, Te, Pb, and Bi in combination. This steel is characterized by its specific inclusions. That is, it contains MnS-type inclusions such that those which have a ratio of major axis to minor axis smaller than 5 account for more than 50% of all. It also contains oxide-type inclusions such that Al2O3 accounts for not more than 15% of all.
Also, Japanese Patent Laid-open No. 23970/1987 discloses a free machining steel which is based on a low-carbon steel made by continuous casting process and incorporated with sulfur and lead. This steel contains C, Mn, P, S, Pb, O, Si, and Al in specific amounts and also contains MnS-type inclusions with a specific average size and sulfide-type inclusions (not combined with oxides) in a specific ratio.
The above-mentioned disclosures are concerned with a free machining steel incorporated with lead and sulfur in combination. There is a tendency in the steel industry toward avoiding the use of lead in reaction to the environmental pollution with lead which is an urgent social issue. Active studies are being made on the improvement of machinability without resort to lead.
Japanese Patent Laid-open No. 87179/2000 discloses a carbon steel or alloy steel for machine structural use which is incorporated with Ca, Mg, and REM (rare earth metal) in combination for superior wear resistance and chip disposability required of machining with a cemented carbide tool. However, it only mentions the composition of sulfide-type inclusions and it does not mention the size of sulfide-type inclusions which has a crucial influence on machinability and mechanical properties.
Japanese Patent Laid-open No. 188853/1995 discloses a carburizing steel for gears which contains 0.0015-0.0350% T.Mg (total Mg) in addition to such basic components as C, Si, Mn, Cr, P, S, T.O (total O). It claims that Mg added to the steel combines with Al2O3 to form MgO.Al2O3 or MgO, making oxide inclusions (mainly alumina) fine, which results in reduction in ductility (due to MnS) and improvement in surface fatigue strength and gear tooth bending fatigue strength. However, it mentions nothing about improvement in impact resistance (in lateral direction) and machinability.
Japanese Patent Laid-open No. 238342/1995 discloses a high-strength carburizing steel for gears which is specified by the content of oxides and sulfides (in terms of number of particles) which satisfies the following conditions.                                           Number            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢                          (                              MgO                +                                                      MgO                    ·                                          Al                      2                                                        ⁢                                      O                    3                                                              )                                            Total            ⁢                          xe2x80x83                        ⁢            number            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            oxides                          ≥        0.80                            (        1        )                                0.20        ≤                              Number            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢                          (                              Mn                ·                Mg                            )                        ⁢            S                                Total            ⁢                          xe2x80x83                        ⁢            number            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            sulfides                          ≥        0.70                            (        2        )            
This steel is an improvement over that disclosed in Japanese Patent Laid-open No. 188853/1995 just mentioned above. The disclosure claims that oxides and sulfides in specific amounts as set forth by (1) and (2) above greatly improve surface fatigue strength and gear tooth bending fatigue strength. However, it mentions nothing about improvement in machinability and impact resistance in lateral direction.
In the meantime, it is known in other field than free-machining steel that oxide-type inclusions, particularly alumina (Al2O3) inclusions, in steel produce such adverse effects as breakage in the case of wire rod such as tire cord, rolling fatigue in the case of bar steel such as bearing quality steel, and cracking at the time of can making in the case of thin steel sheet for DI process. For alleviation of these adverse effects, several attempts were made to reduce the amount of alumina-type inclusions. One way disclosed in Japanese Patent No. 2140282, for example, is to add a Mg alloy to a molten steel containing Si, Mn, Al, and C, thereby preventing Al2O3 in the steel from becoming coarse through aggregation. Mg added to a molten steel converts Al2O3 into MgO.Al2O3 which is fine particles having no adverse effect on the steel.
Also, Japanese Patent Laid-open No. 225822/1996 discloses an improvement on a steel containing Al and S by sequential addition of Ca and Mg. These additives convert alumina inclusions in the steel into a binary oxide (CaOxe2x80x94Al2O3) or a ternary oxide (CaOxe2x80x94Al2O3xe2x80x94MgO), which has a lower melting point. To be more specific, upon addition of Ca and Mg to a molten steel, such inclusions as Al2O3 and CaS which cause nozzle clogging change into compound oxides having a lower melting point than 12CaO.7Al2O3, without forming CaS in an appreciable amount. The steel modified in this way is free from nozzle clogging. The above-mentioned method is applied to an Al-killed steel to prevent Al2O3 from becoming coarse through aggregation. Therefore, the molten steel already contains Al before incorporation with Mg.
In addition, Japanese Patent No. 2684307 discloses a method of efficiently preventing Al2O3 from aggregation in a molten steel by addition of an Mgxe2x80x94Al alloy to a molten steel containing Si, Mn, and C. Adding Mg and Al simultaneously in the form of alloy permits efficient and rapid reactions. The result is an improved yield per unit amount of Mg added. Unfortunately, Mg readily vaporizes and hence does not remain as much as Al in the molten steel when Mg and Al are added simultaneously. Consequently, Al2O3 is much more prone to occur, creating a state very similar to that which would be if Al is added first. In other words, Mg added simultaneously with Al is not so effective in dispersing inclusions in the form of fine particles.
As mentioned above, attempts made so far to improve machinability are based mainly on controlling the size and shape of sulfide-type inclusions (such as MnS) in resulfurized carbon steel. However, none of free-machining steel has been realized which is comparable to leaded carbon steel. Moreover, any attempt to control the size and shape of sulfide-type inclusions causes MnS particles to elongate as the base metal (steel) undergoes plastic deformation during rolling or forging. The elongated MnS particles cause mechanical anisotropy, with the result that the steel has a lower impact value in one direction than in other directions.
Now, machinability is rated in terms of (1) cutting resistance, (2) tool life, (3) finished surface roughness, and (4) chip disposability. In the past, importance was attached to the second and third items. However, the fourth item has recently become important from the standpoint of working efficiency and safety as automated or unmanned machining has become common. Chip disposability is an ability of steel to become small chips after cutting. With poor chip disposability, a work tends to give rise to long coiled chips which entangle with the cutting tool. As long as chip disposability is concerned, the conventional lead-containing free-cutting steel is satisfactory; however, so far there is no lead-free steel having good chip disposability.
The present invention was completed in order to address the above-mentioned problems. It is an object of the present invention to provide a machine structure steel and a process for production thereof, said steel being superior in chip disposability and mechanical properties despite its substantial freedom from lead.
The present invention is directed to a machine structure steel superior in chip disposability and mechanical properties which contains sulfide-type inclusions such that those particles of sulfide-type inclusions with major axes in a specific range have a controlled average aspect ratio and which also contains coarse particles of sulfide-type inclusions in a limited number.
To be more specific, the gist of the present invention resides in a machine structure steel superior in chip disposability and mechanical properties which contains sulfide-type inclusions such that those particles of sulfide-type inclusions with major axes not shorter than 5 xcexcm have an average aspect ratio not larger than 5.2 and which also contains coarse particles of sulfide-type inclusions such that the following relation is satisfied.
a/bxe2x89xa60.25
where, a denotes the number of particles of sulfide-type inclusions with major axes not shorter than 20 xcexcm, and b denotes the number of particles of sulfide-type inclusions with major axes not shorter than 5 xcexcm.
The aspect ratio in the present invention is defined as c/d, where c and d respectively denote the major axis and minor axis of a particle of sulfide-type inclusions. The major axis of a particle is defined as the diameter of the maximum circle circumscribing the particle. The minor axis of a particle is defined as the maximum width of the particle measured in the direction perpendicular to the diameter of the maximum circle.
According to a preferred embodiment, the machine structure steel of the present: invention satisfies the condition that [Mg]/[S]xe2x89xa77.7xc3x9710xe2x88x923 (where [ ] denotes the content (mass %) of each component), those particles of sulfide-type inclusions with major axes not shorter than 50 xcexcm have an average aspect ratio not larger than 10.8, and a/bxe2x89xa60.25 (where a and b are defined as above).
According to another preferred embodiment, the machine structure steel of the present invention satisfies the condition that ([Mg]+[Ca])/[S]xe2x89xa77.7xc3x9710xe2x88x923 (where [ ] denotes the content (mass %) of each component), those particles of sulfide-type inclusions with major axes not shorter than 50 xcexcm have an average aspect ratio not larger than 10.8, and a/bxe2x89xa60.25 (where a and b are defined as above).
According to another preferred embodiment, the machine structure steel of the present invention contains 0.01-0.7% C, 0.01-2.5% Si, 0.1-3% Mn, 0.01-0.16% S, not more than 0.05% P (0% inclusive), not more than 0.1% Al (0% inclusive), and not more than 0.02% Mg (0% not inclusive). It may additionally contain not more than 0.02% Ca (0% not inclusive) and not more than 0.3% Bi (0% not inclusive). xe2x80x9c%xe2x80x9d means xe2x80x9cmass %xe2x80x9d, and the same shall apply herein after.
The present invention is also directed to a process for producing a machine structure steel, said process comprising a step of adding a substantially Al-free Mg alloy to a substantially Al-free molten steel. This process may be modified such that addition of said Mg alloy is followed by addition of Al.
The present invention is also directed to a process for producing a machine structure steel, said process comprising a step of adding a substantially Al-free Mg alloy and a subsequent step of adding a substantially Al-free Ca alloy to a substantially Al-free molten steel. This process may be modified such that addition of said Ca alloy is followed by addition of Al.
The present invention is also directed to a process for producing a machine structure steel, said process comprising a step of adding a substantially Al-free Mg alloy and a substantially Al-free Ca alloy all together as many times as necessary to a substantially Al-free molten steel, or said process comprising a step of adding a substantially Al-free Mg alloy earlier than a substantially Al-free Ca alloy and then adding these two alloys in any order as many times as necessary. This process may be modified such that addition of said Mg alloy and said Ca alloy is followed by addition of Al.
The above-mentioned process may be carried out efficiently if the molten steel is covered with a slag containing 15% or more MgO.